A radio communication system is a type of communication system in which a transmitter and receiver are connected by a communication channel defined upon a portion of the electromagnetic frequency spectrum. Fixed or hard-wired connections are not required to form a communication channel to interconnect the transmitter and receiver. Communications can be effectuated between the transmitter and receiver in a radio communication system even when the use of a fixed or hard-wired connection would be inconvenient or impractical.
Technological advancements in communication technologies have permitted the widespread utilization of multiple-access, cellular communication systems. Installation of the network infrastructure of conventional, terrestrial cellular communication systems permits multiple numbers of users to communicate by way of the terrestrial cellular system when positioned in an area encompassed by such a system. A terrestrial cellular system is constructed, typically, pursuant to a selected standard specification. To be operable to communicate by way of a terrestrial cellular system, a radiotelephone positioned within the geographical area encompassed by the system must be constructed to permit its operation in such system.
Additional technological advancements have permitted the workability of multiple-access satellite communication systems. And, various proposals have been set forth for satellite communication systems. A satellite communication system is generally able to provide for wider-area geographic coverage than that of a conventional terrestrial cellular system. And, compatibility problems resulting from positioning of a radiotelephone operable in only one terrestrial cellular communication system in a geographical area encompassed by a different type of terrestrial system is therefore less likely to occur. Various ones of the proposed satellite communication systems, for instance, permit almost worldwide coverage.
At least one type of satellite communication system proposes to make use of geostationary satellites which shall provide communication coverage over fixed local areas. Another proposal for a satellite communication system shall make use of low-earth, orbiting systems having numerous satellites placed in low-earth orbits and providing coverage over large areas of, or even entirely encompassed, the earth. Proposed satellite communication systems include the Iridium system utilizing sixty-six low-earth orbiting satellites, the intermediate circular orbit (ICO-P21) system having twelve satellites positioned in a medium-earth orbit, and the ASEAN cellular satellite system (ACeS) using a geostationary or a geosynchronously-positioned satellite to provide local communication coverage over a selected portion of the earth.
Dual-or multi-mode radiotelephones have been proposed which would permit communication alternately pursuant to a satellite communication system and pursuant to a terrestrial cellular communication system. Such a phone would permit user-selection of the communication system through which communications are to be effectuated.
Generally, the proposed satellite communication systems shall provide the ability to communicate both voice and data. A user having a radiotelephone, also referred to herein as a "user terminal", operable to communicate by way of the satellite communication system shall be able to communicate therethrough when positioned at almost any location.
In order to effectuate communications between the user terminal and a satellite network, the radiotelephone must be synchronized to the satellite communication network. A beacon signal transmitted by satellite-positioned transceivers forms a synchronization signal to which a radiotelephone can be synchronized. In the afore-mentioned ACeS system, an HPS (high power synchronization) signal is generated. The HPS signal is formed of four uniquely-spaced, high power bursts. Such bursts are repeated during each multi-frame of data according to the standard specification of the ACeS system. Three of the high power bursts of the HPS signal contain broadcast information. And, a fourth of the high power bursts contains a maximal-length, pseudo-random sequence.
In the ACeS satellite communications system, signals are directed in selected ones of one hundred forty-four different beams. Each of such beams is assigned to a unique HPS frequency. A plurality of HPS carriers are allocated in the ACeS satellite communication system, appropriately distributed amongst the one hundred forty-four beams in a manner which minimizes interference between signals directed to adjacent geographical areas. And, in some jurisdictions, the frequencies allocated for transmission of the signals vary from those frequencies allocated in other jurisdictions. The radiotelephone might be positioned in an area in which the HPS carrier allocation is not known. As a result, a search must be made over an entire frequency band. The HPS carriers may, for example, number about thirty.
The HPS signal, when received at the radiotelephone, may be of a low signal energy level, particularly when the radiotelephone is not positioned in a manner to facilitate its detection of the HPS signal. For instance, the radiotelephone might be positioned in a pocket or purse of a user.
The HPS signal, for instance, may be received at a signal-to-noise ratio (SNR) on the order of -10 dB. At such a low SNR, detection of, and synchronization to, the HPS signal within a desirably small time period is difficult.
A synchronization method exists which facilitates the synchronization of a radiotelephone when the carrier upon which the HPS signal is transmitted is known. Synchronization may, however, require as long as three seconds per carrier. While such existing manner by which to synchronize the user terminal to the HPS signal is efficient in terms of power consumption, the amount of time required to synchronize the radiotelephone to the HPS signal in the existing art is sometimes too lengthy. The amount of time required to synchronize a radiotelephone might require up to ninety seconds of processing time prior to completion of the acquisition.
As the amount of processing time required to detect and synchronize the radiotelephone to the HPS signal is directly related to the processing power, a reduction in the amount of time required to detect and synchronize the radiotelephone thereto would also provide additional power savings, thus increasing standby time.
A manner by which to detect, and thereafter synchronize a radiotelephone to a synchronization signal in a reduced amount of time would be advantageous.
It is in light of this background information related to radio communication systems that the significant improvements of the present invention have evolved.